Step 1In a small bowl, whisk together the soy sauce and mirin.
Step 1Brush the onigiri with a little oil to prevent it from sticking to the grill. If you use a spicy oil like chili oil, it will give the onigiri heat.
Step 2Heat a grill pan or grill over medium-high heat until hot, or heat the broiler. Line the grill pan, grill or a baking sheet (if using the broiler) with foil. Grill the onigiri on both sides until crisp and slightly toasted; this can take from 5 to 10 minutes on each side depending on the heat and cooking method. While grilling, baste the onigiri with a little of the sauce on each side a few times until it is absorbed and becomes crisp; the onigiri should not be moist from basting when done. Watch carefully, as the onigiri can burn.
Step 3Serve immediately while the onigiri are piping hot.